The recycling of copper from copper-containing scrap accounts for a significant part of the United States copper supply. While scrap with a copper concentration greater than 50% may be economically smelted, lower grades of copper scrap, particularly those with a copper concentration of less than 30% by weight, are infrequently recycled because existing methods are not economical. Low grade copper scrap is generated at a rate of 200,000-300,000 tons per year. An example of low grade scrap is the "breakage" portion of shredded automobile scrap produced by the sorting of copper-containing motors. The breakage typically contains 10-20% copper by weight.
Recycled copper can be used in the production of valuable copper compounds. Copper compounds such as tribasic copper sulfate and copper sulfate have wide application. Copper sulfate is a fungicide and algicide and is used as a source of copper in animal nutrition, as a fertilizer, and as a source for other copper compounds. Tribasic copper sulfate is a fungicide and is a source for other copper compounds.
In view of the large quantity of low grade copper scrap and the commercial value of copper compounds, an economical method for the production of copper from low grade copper materials is needed. Preferably the method would produce the copper in a concentrated form with minimal contamination by other metals.
Cupric tetrammine sulfate has long been regarded as a desirable lixiviant for the selective extraction of copper from copper-containing ores, dust and scrap. The resulting copper ammine sulfate leachate can then be used as a electrolyte in the production of cathode copper. However, contaminants can be present in the leachate/electrolyte in quantities sufficient to contaminate the copper plate at the cathode. Common contaminants found in leachate produced from the leaching of ores, dust and scrap include zinc and nickel. During the process of leaching copper-containing ores, dusts or scrap, the cupric ion of the lixiviant undergoes an oxidation/reduction reaction with the copper metal in the scrap, ore or dust to form the soluble cuprous ion. However, the cupric ion also undergoes oxidation/reduction with nickel and zinc in the scrap, ores or dust to form soluble nickel and zinc ammine sulfates. The resulting leachate contains solubilized copper ammine sulfate, zinc ammine sulfate and nickel ammine sulfate. To maintain a 99.99% by weight purity of copper plate, the nickel concentration should remain below about 10 grams per liter (g/l) and the zinc concentration should remain below about 25 g/l in the leachate/electrolyte. Accordingly, there is a need for a method to reduce nickel and zinc contamination in copper ammine sulfate leachates to allow the cathode recovery of high purity copper. Preferably, this method would remove the nickel and zinc in a form that would allow nickel and zinc recovery as well.
Methods have been devised for the recovery of zinc and copper from ammoniacal ammonium sulfate leachates. In Stern et al., U.S. Pat. No. 3,929,598 (1975), a method is disclosed for the removal of copper from an ammoniacal ammonium sulfate leachate by treatment with a liquid ion exchanger. Liquid ion exchange treatment comprises an organic, water-immiscible solution of a reagent which selectively loads copper in countercurrent relation with the copper and zinc ammonium sulfate leachate. The loaded organic phase is then stripped with an aqueous solution of sulfuric acid to produce a copper sulfate solution substantially free of nickel, zinc and cobalt contamination, that can be recovered by electrowinning. The aqueous ammonium sulfate raffinate contains zinc and may contain nickel, cobalt, silver and minor amounts of copper. Depending on the level of nickel, cobalt, silver and copper contamination of the zinc ammonium sulfate raffinate, the raffinate may be used directly as an electrolyte for the recovery of zinc, or may undergo a variety of purification methods prior to the electrowinning of zinc. A disadvantage of the Stern et al. method is that it does not provide for the recovery of nickel from the nickel, zinc and copper ammine sulfate leachate.
Ullrich et al., U.S. Pat. No. 3,523,787 (1970), discloses a method for the recovery of copper and zinc from an ammoniated ammine sulfate leachate by the precipitation of a copper-zinc ammonium sulfate salt. To form the precipitate, the pH of an ammoniated copper and zinc ammonium sulfate leachate is reduced to between pH 1.6 and pH 3.0. The precipitate is then separated from the leachate and heated to 400.degree. to 500.degree. C. to volatilize water and ammonia. The resulting solid is a mixture of anhydrous copper and zinc sulfates and oxides, which are then taken up in a dilute sulfuric acid slurry to separate contaminating lead and calcium in the form of insoluble PbSO.sub.4 and CaSO.sub.4. Following removal of the PbSO.sub.4 and CaSO.sub.4 precipitate, the high purity copper-zinc sulfate brine is treated with potassium chloride to form CuCl.sub.2 and ZnCl.sub.2. Cupric chloride (CuCl.sub.2) is then reduced by bubbling through SO.sub.2 gas to form insoluble cuprous chloride (CuCl). The removed CuCl can then be reduced to produce metallic copper of high purity. Following removal of the CuCl precipitate, the ZnCl.sub.2 brine, which may contain some residual copper, is purified by zinc cementation to remove the copper. The purified ZnCl.sub.2 brine is then treated with NaCO.sub.3 to cause the precipitation of ZnCO.sub.3. The Ullrich et al. method has the disadvantage that it does not provide for the recovery of nickel.
Accordingly, it would be advantageous to have a method for decreasing the nickel and zinc concentrations in a copper ammine sulfate solution. The method preferably should provide for the removal of zinc and nickel in a form that allows for zinc and nickel recovery. It has been found that the present invention meets these needs.